Inflatable restraint cushions are commonly installed in motor vehicles to reduce the likelihood of the vehicle occupants sustaining injuries during vehicle crashes. Inflatable restraint cushions are commonly known as airbags in the safety restraint industry. During a vehicle crash, an airbag is rapidly filled with inflation gas between the vehicle occupant and the interior of the vehicle. The airbag absorbs the vehicle occupant's kinetic energy to provide a controlled, reduced deceleration of the vehicle occupant and to prevent the vehicle occupant from contacting the hard surfaces of the vehicle interior.
The source of inflation gas for an airbag is an inflator. An inflator may provide inflation gas in a number of manners such as through burning of pyrotechnic material (pyrotechnic inflator), through releasing stored gas (cold gas inflator), or through some combination thereof (hybrid inflator). Each of the above mentioned manners are well known in the art.
The pyrotechnic inflator typically includes an igniter, an enhancer (also known as a booster), and a gas generant. During the car crash, the igniter is fired, which ignites the enhancer, which in turn ignites the gas generant. The burning of the gas generant produces gaseous combustion products useful for inflating a vehicle airbag. Also well known in the art are dual stage pyrotechnic inflators, which have two combustion chambers. The dual stage inflators typically have two igniters for igniting gas generant in each of the two combustion chambers. The advantage of utilizing a dual stage inflator is its tailorability. There are numerous firing scenarios for a dual stage inflator such as, the firing of only the first stage, the firing of the first stage followed by a delayed firing of the second stage, and the firing of the first stage and second stage simultaneously.
The gas generant useful for a pyrotechnic inflator is a blend of a fuel and an oxidizer. The combustion of the fuel and oxidizer produces gaseous combustion products. Higher yield gas generants are desirable for a couple of reasons. First, higher yield gas generants require less gas generant to produce the same gas output as lower yield gas generants. Second, higher yield gas generants produce less solid particles or slag that need to be filtered. Organic compounds rich in nitrogen are typically selected for the fuel. Well-known oxidizers useful as oxidizers in gas generants are strontium nitrate and potassium nitrate. Another well-known oxidizer, ammonium nitrate, is desirable because the use of ammonium nitrate increases the gas yield of the gas generant because metal ions are not present in the oxidizer. Gas generants having a high nitrogen content organic molecule and ammonium nitrate have gas yields over 95%. As used herein, ammonium nitrate gas generant refers to a gas generant that contains a fuel and ammonium nitrate as the oxidizer.
The downside of using an ammonium nitrate gas generant is the low melting temperature of the eutectics or fuel/oxidizer mixture. The low temperature property of ammonium nitrate gas generants becomes a serious issue in obtaining shipping classification from the United States Department of Transportation. To obtain shipping approval, the inflator needs to maintain structural integrity or in other words should not fragment when an autoignition material in the inflator is induced by external heating (i.e. bonfire test). The autoignition material is a pyrotechnic mixture that spontaneously combusts or autoignites at a temperature before the gas generant undergoes a physical or chemical change such as decomposition, autoignition, or melting. Thus, in order for the autoignition material to function properly, the autoignition material needs to spontaneously ignite below a temperature that results in a chemical or physical change in the gas generant. Typically, in the industry the autoignition material needs to spontaneously ignite at a temperature above 130° C. If the autoignition material ignites after the gas generant undergoes a physical or chemical change, then the ballistic properties of the gas generant become unpredictable and the structural integrity of the inflator may fail.
As discussed earlier, ammonium nitrate is a highly desirable oxidizer for gas generants because this oxidizer increases the conversion rate of the gas generant to gaseous combustion products. Ammonium nitrate melts at about 169° C., and the addition of a fuel to the oxidizer may result in a eutectic that has a lower melting point. If the fuel is nitroguanidine or guanidine nitrate the resulting eutectic (fuel and oxidizer) has a melting point at about 135° C. If the fuel is 5-amino tetrazole, then the eutectic has a melting point as low as 115° C. Thus, a gas generant having ammonium nitrate and either nitroguanidine or guanidine nitrate has a melting point extremely close to 130° C., the lower autoignition temperature limit on the autoignition material. In order for the autoignition material to effectively deploy the gas generant, the autoignition material needs to be ignited before the gas generant melts. Accordingly, there is a desire to design an inflator that overcomes the potential melting of the ammonium nitrate gas generant before the ignition of the autoignition material.